As your storage card fills up, the computer can't store the file as one block (that's called contiguous storage), it has to find empty space that is distributed randomly inside it to store the files in parts, and that slows down the write speed considerably. This is like going to the threatre with a group and it's pretty full. You can't all sit together, so you have to spend time finding empty seats! 
The process as regards using your card with your computer goes as follows:
Copy all the material: videos and stills from the SD card to your computer.
If you use WIndows 10, Use the eject feature* to remove the card, don't just pull it out, you can permanenlty kill the card doing that.
Put the card back into your camera
Format the card IN THE CAMERA - don't just delete the files, that's another way for corrupting the card.
* Windows 10 Eject Feature
SD cards should only be used to copy material to the computer's hard disc, do NOT attempt to process material directly from the card (it's slow and can cause errors when you go to remove the card0. Once you have copied the material, Eject the card.
In Windows 10 there are two ways to eject a card.
From file manager, make sure your default location is not the card. Also make sure no other programs are using the card.
In the left-hand window (where the file structure is displayed, locate the card, RIGHT+CLICK on the ICON for the card and a pop-up menu should appear. On that menu LEFT+CLICK on the Eject feature. If you are successful the icon for the card will disapper, you can then take the card out of the computer.
If that doesn't work, close File Manager and go to the tray on the bottom right of the desktop. Click on the ^ symbol to bring up a menu. On that you will see a symbol as shown below - which is the Eject command icon, Click on that and the drive description for the card will be shown. Click on that and the item should disapper off the list. You can then remove the card.
I think there is a similar system for Macs.
